Have you ever thought about how a robotic
vehicle commonly used in military application with bomb detention is
controlled or how metal cutting and forming machines provide
precise motion for milling, lathes and bending for metal fabrication or how an antenna positioning system control
the precision in azimuth and elevation? As you will learn within this lesson, servo motor applications are most
commonly used in closed loop systems where precise position control commonly found
in industrial and commercial applications. Together with the recently
RealPars published video, What is a Stepper Motor and How it Works, and this lesson, you will learn about motion
control using different types of motors available, primarily stepper and servo motors. Before we get into today’s
video, if you love our videos, be sure to click the like button below, and make sure to click subscribe and the bell to
receive notifications of new RealPars videos. This way you never miss another one! In this lesson we will discuss what
a servo motor is and how it works, so let’s first determine what a servo motor is and examine some unique features of the
types of a servo motor and its application. Servo Motor Basics Let’s begin, with the servo motor basics. Servo motors are part
of a closed-loop system and are comprised of several
parts namely a control circuit, servo motor, shaft, amplifier and either an encoder or resolver. A servo motor is a self-contained
electrical device, that rotate parts of a machine with high
efficiency and with great precision. The output shaft of this motor can
be moved to a particular angle, position and velocity that a
regular motor does not have. The Servo Motor utilizes a regular motor and couples it with a sensor
for positional feedback the controller is the most
important part of the Servo Motor designed and used specifically
for this purpose. The servo motor is a closed-loop mechanism
that incorporates positional feedback in order to control the rotational
or linear speed and position. The motor is controlled with an electric
signal, either analog or digital, which determines the
amount of movement which represents the final
command position for the shaft. A type of encoder serves as a sensor
providing speed and position feedback. This circuitry is built right
inside the motor housing which usually is fitted with gear system. Types of Servo Motors are classified into
different types based on their application, such as the AC servo
motor, and DC servo motor. There are three main considerations
to evaluate servos motors, first based on their current type - AC or DC, and secondly on the type
of Commutation used, whether the motor uses brushes and the third type
of consideration is the motors rotating field, the rotor, whether the rotation
is synchronous or asynchronous. Let’s discuss the first servo consideration. AC or DC consideration is the most
basic classification of a motor based on the type of current it will use. Looking at it from a
performance standpoint, the primary difference
between AC and DC motors is in the inherit ability to control speed. With a DC motor, the speed
is directly proportional to the supply voltage with a constant load. And in an AC motor, speed is determined
by the frequency of the applied voltage and the number of magnetic poles. while both AC and DC motors
are used in servo systems, AC motors will withstand higher current and are more commonly used
in servo applications such as with robots, in-line manufacturing and other industrial applications
where high repetitions and high precision are required. Brushed or brushless is the next step. A DC Servo Motor is commutated
mechanically with brushes, using a commutator, or
electronically without brushes. Brushed motors are generally less
expensive and simpler to operate, while brushless designs
are more reliable, have higher efficiency, and are less noisy. A commutator is a rotary
electrical switch that periodically reverses
the current direction between the rotor and the drive circuit. It consists of a cylinder composed of
multiple metal contact segments on the rotor. Two or more electrical
contacts called "brushes" made of a soft conductive material such
as carbon press against the commutator, making a sliding contact with segments
of the commutator as it rotates. While the majority of motors used in
servo systems are AC brushless designs, brushed permanent
magnet DC motors are sometimes employed
as servo motors for their simplicity and low cost. The most common type of
brushed DC motor used in servo applications is the
permanent magnet DC motor. Brushless DC motors replace
the physical brushes and commutator with an electronic
means of achieving commutation, typically through the use of Hall
effect sensors or an encoder. AC motors are generally brushless, although there are some
designs - such as the universal motor, which can run on either AC or DC power, that do have brushes and are
mechanically commutated. And the final classification to consider is whether the servo motor application will use a synchronous or
asynchronous rotating field. While DC motors are generally
categorized as brushed or brushless, AC motors are more often differentiated by the speed of their rotating
synchronous or asynchronous field. If we recall from the AC-DC consideration, that in an AC motor, speed is determined
by the frequency of the supply voltage and the number of magnetic poles. This speed is referred to
as the synchronous speed. Therefore, in a synchronous motor, the rotor rotates at the same speed as the stator’s rotating magnetic field. However, in an asynchronous motor, normally referred to as an induction motor, the rotor rotates at a speed slower than
the stator’s rotating magnetic field. However, the speed of
an asynchronous motor can be varied utilizing
several control methods such as changing the number of poles, and changing the frequency
just to name a couple. The working principles of a DC servo motor are the construction of
four major components, a DC motor, a position sensing device,
a gear assembly, and control circuit. The desired speed of the DC motor
is based on the voltage applied. In order to control the motor speed, a potentiometer produces a voltage which is applied as one of the
inputs to error amplifier. In some circuits, a control pulse is
used to produce DC reference voltage corresponding to desired
position or speed of the motor and it is applied to a pulse
width voltage converter. The length of the pulse decides the
voltage applied at the error amplifier at the error amplifier as a desired voltage
to produce the desired speed or position. For digital control, a PLC
or other motion controller is used for generating the pulses in terms of
duty cycles to produce more accurate control. The feedback signal sensor
is normally a potentiometer that produces a voltage corresponding to the absolute angle of the motor
shaft through the gear mechanism. Then the feedback voltage value is applied
at the input of error comparator amplifier. The amplifier compares the voltage
generated from the current position of the motor resulting from
the potentiometer feedback and to the desired position of the motor producing an error either of a
positive or negative voltage. This error voltage is applied
to the armature of the motor. As the error increases so does the output
voltage applied to the motor armature. As long as error exists,
the comparator amplifier, amplifies the error voltage and
correspondingly powers the armature. The motor rotates until
the error becomes zero. If the error is negative, the
armature voltage reverses and hence the armature rotates
in the opposite direction. The working principles
of an AC servo motors are based on the construction with two
distinct types of AC servo motors, they are synchronous and
asynchronous (induction). The synchronous AC servo motor
consist of stator and rotor. The stator consists of a
cylindrical frame and stator core. The armature coil wound
around the stator core and the coil is connected to a lead wire through
which current is provided to the motor. The rotor consists of a permanent magnet and this differs with the
asynchronous induction type rotor in that the current in the rotor
is induced by electromagnetism and therefore these types are
called as brushless servo motors. When the stator field is
excited with voltage, the rotor follows the rotating
magnetic field of the stator at the same speed or synchronized
with the excited field of the stator, and this is where the
synchronous type is derived. With this permanent magnet rotor, no rotor current is required so when the stator
field de energizes and stops, the rotor also stops. These motors have higher efficiency
due to the absence of rotor current. When the position of rotor with
respect to stator is required an encoder is placed on the rotor and provides
feedback to the servo motor controller. The asynchronous or induction AC servo
motor stator consists of stator core, armature winding and lead wire and the rotor consists of shaft
and the rotor core winding. Most induction motors contain a rotational
element, the rotor or squirrel cage. Only the stator winding
is fed with an AC supply. Alternating flux field is produced around
the stator winding with the AC supply. This alternating flux field
revolves with synchronous speed. The revolving flux is called a
rotating magnetic field (RMF). The relative speed between
stator rotating magnetic field and rotor conductors causes an
induced electromagnetic force in the rotor conductors according to
Faraday's law of electromagnetic induction. This is the same action that
occurs in transformers. Now, the induced current in rotor will also produce an alternating
flux field around itself. This rotor flux lags
behind the stator flux. The rotor velocity is related between
the rotating stator flux field and the rotor rotates in the same
direction as that of the stator flux. The rotor does not succeed in
catching up the stator flux speed or not synchronized, hence where
the type asynchronous is derived. Servo Motor Applications are applied in
many industrial and commercial systems and products such as with robotics where a servo motor is used
at every "joint" of a robot to perform its precise angle of movement. The camera auto focus uses a servo
motor built into the camera that corrects precisely
the position of lens to sharpen the out-of-focus images. And with antenna positioning systems where servo motors are used for
both the positioning of azimuth and elevation axis of
antennas and telescopes such as those used by the National
Radio Astronomy Observatory. This concludes the video, what is
a Servo Motor and How it Works. I hope you have learned what’s
required to move forward in creating your own
motion control project. If you enjoyed this video,
please press the like button. This video is one of a
series of videos on motor motion control, so please check back with
us soon for more motion control topics. Make sure that you head
over to realpars.com. To find even more training material
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